THE CIRCULATION 161 



patient's ' blood-pressure.' This distinction between the 

 systolic and diastolic pressure has already been pointed 

 out when the pulse was described, and it was then seen 

 that the systolic pressure depends mainly upon the left 

 ventricle and the diastolic pressure upon the degree 

 of contraction, of the peripheral arterioles. An estima- 

 tion of the former, therefore, gives us most information 

 about the heart ; an estimation of the diastolic pressure 

 about the tone of the arterioles. Information on both 

 of these points is of great use clinically, but whereas we 

 have many other means of ascertaining the efficiency of 

 the heart, we have none of estimating the degree of peri- 

 pheral constriction ; it is, therefore, to be regretted that 

 our clinical methods of measuring diastolic pressure are 

 not more perfect. 



The factors concerned in maintaining blood-pressure 

 are : 



1. The Force and Output of the Left Ventricle. 



If the force fails or the output per beat diminishes, 

 blood-pressure falls. 



2. The peripheral resistance, which is made up of 

 the opposition offered to the passage of the blood by the 

 tonic contraction of the small arteries, by the friction 

 produced in the capillaries by the viscosity of the blood, 

 and by the ' tissue- tension.' * Variations in the degree of 



* Under the term ' tissue-tension ' may be included not only 

 the elastic pressure exercised on the vessels from without, but the 

 atmospheric pressure, and all casual mechanical pressures the 

 result of posture, etc. That these, collectively, are a considerable 

 factor in making up the sum total of the ' peripheral resistance ' 

 there can be no doubt. The influence of external pressure, for 



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